The reason galaxies do not fall into their central black holes and remain stable over long periods of time is primarily due to the dynamics of their orbits and the balance of forces involved. Here are a few key factors that contribute to the stability of galaxies:
Orbital dynamics: Galaxies are composed of billions to trillions of stars, along with gas, dust, and other celestial objects. These components orbit around the central black hole, forming a complex gravitational dance. The combined gravitational attraction between the stars and the central black hole creates stable, well-defined orbits, preventing the stars from spiraling directly into the black hole.
Conservation of angular momentum: Stars and other objects in galaxies possess angular momentum, which is a property related to their rotation and orbital motion. Conservation of angular momentum dictates that this property remains constant unless acted upon by external forces. As a result, the stars in a galaxy maintain their orbits and do not simply collapse into the central black hole.
Stellar dynamics and collisions: While individual stars can come close to the central black hole, stellar interactions and collisions within galaxies can redistribute their energy and angular momentum. This process prevents most stars from directly falling into the black hole. Instead, they continue to orbit the galactic center.
Feedback mechanisms: Galaxies often exhibit active galactic nuclei (AGNs) associated with their central black holes. These AGNs can release immense amounts of energy in the form of jets, radiation, and outflows. These energetic processes can regulate the growth of the black hole and also influence the surrounding interstellar medium. The feedback from AGNs can help stabilize the galaxy by counteracting gravitational collapse and regulating star formation.
It's important to note that while galaxies remain stable on long timescales (billions of years), they can still undergo interactions and mergers with other galaxies, which can alter their structure and dynamics. However, the overall stability and persistence of galaxies over vast cosmic timescales can be attributed to the delicate balance between gravitational forces, angular momentum, stellar dynamics, and feedback mechanisms from the central black hole.